Method of nuclear quadrupole resonance testing of integral spin quantum number systems

ABSTRACT

A method of Nuclear Quadrupole Resonance testing includes the steps of applying to a sample in which selected nuclei have an integral spin quantum number a series of at least three pulses of electromagnetic waves at a single radiofrequency to excite quadrupole resonance of the selected nuclei, and detecting responses at a plurality of times when echo response signals are expected to occur.

FIELD OF THE INVENTION

The present invention relates to a method of nuclear quadrupoleresonance (NQR) testing of integral spin quantum number spin systems.

DESCRIPTION OF RELATED ART

NQR testing is used for detecting the presence or disposition ofspecific substances. It depends on the energy levels of quadrupolarnuclei, which have a spin quantum number greater than one-half.Quadrupolar nuclei having an integral spin quantum number (that is, I=1,2, 3 . . . ) include ¹⁴ N (I=1). ¹⁴ N nuclei are present in a wide rangeof substances, including animal tissue, bone, food-stuffs, explosivesand drugs.

In the sub-molecular environment of compounds or crystals, the natureand disposition of the electrons and atomic nuclei produce an electricfield gradient which modifies the nuclear energy levels to such anextent that measurements of NQR effects can indicate not merely thenuclei which are present but also their chemical environment, thusindicating specific substances or types of substances in any testedsample.

In NQR testing a sample is irradiated with pulses or sequences of pulsesof radiofrequency electromagnetic waves having a frequency which is ator very close to a resonant frequency of quadrupolar nuclei in asubstance which is to be detected. If the substance is present, theirradiant energy will raise at least some of the nuclei to a higherenergy level. Such nuclei will tend to return to their normal state andin doing so they will emit radiation at their resonance frequency orfrequencies which can be detected as a free induction decay (f.i.d.)during a decay period after each pulse. These emissions decay at a ratewhich depends on two relaxation time constants, T₁ and T₂.

In conventional NOR testing, either a substantial part of the freeinduction decay is measured after each pulse or the responses aremeasured as echoes in relatively short sampling periods between orfollowing a relatively rapid succession of pulses. Usually the resultsfrom a number of test pulses or test sequences are integrated to improvethe signal-to-noise ratio. Various schemes of pulse sequences have beenused.

In a scientific paper by Grechiskin et al. (Adv. N.Q.R., 1983, 5, 1) ithas been predicted theoretically that conditions might arise which couldgive rise to the formation of a single echo as well as a free inductiondecay from nuclei of unity spin quantum number when excited by twoexcitation pulses at a single radiofrequency. No details are given as tohow, or even whether, this can be achieved experimentally.

In a paper by Bloom et al. (Physical Review 1955, vol. 97, 1699) it hasbeen reported that multiple echoes as well as a free induction decayhave been observed in tests from nuclei of spin quantum number 3/2 in aweak magnetic field. The paper demonstrates that the magnetic fieldremoves degeneracies which would otherwise so broaden and attenuate theechoes that no useful NOR information would be yielded. However, thistechnique would not be expected to work for integral spin systems, sincesuch systems are not strongly affected by a wear applied magnetic field.

SUMMARY OF THE INVENTION

According to the present invention, a method of NQR testing includes thesteps of applying to a sample in which selected nuclei have an integralspin quantum number a series of at least three excitation pulses ofelectromagnetic waves at a single radiofrequency to excite quadrupoleresonance of the selected nuclei and detecting responses at a pluralityof times when echo response signals are expected to occur, theexcitation pulses being arranged to generate more echo response signalsthan the number of applied pulses.

We have found that, surprisingly, for samples in which the selectednuclei have an integral (e.g. unity) spin quantum number, in addition tothe free induction decay which occurs immediately after a singlefrequency excitation pulse, there are some substantial NQR echoresponses which occur after delays matching the time intervals betweenpreceding pulses. It has been found that, if time τ is measured from thefirst pulse of a series of pulses of pre-selected widths and phasesoccurring at times 0, τ₁, τ₂ . . . τ_(N) which are not necessarilyidentically spaced there are substantial echo responses at many,although not all of, the following times: ##EQU1## where n=1 to N. Ithas also been found that there can be echo responses which occur after adelay which matches the time interval between an excitation pulse and aecho response which precedes it, for instance at times

    τ.sub.n +(τ.sub.n -2τ.sub.1)=2τ.sub.n -2τ.sub.1.

The times when echo response signals would be expected to occur for aparticular substance or class of substance can be predetermined byexperimentation in a manner which will be evident to a person skilled inthe art.

It will be appreciated that, since more (often many more) echoes may bedetected from the sample than excitation pulses are applied to thesample, and since the usual f.i.d.'s may also be detected, a largersignal-to-noise ratio can be obtained than would be obtained for asystem in which a multiple echo train was not generated; the sensitivityof the tests can thus be improved. Therefore, it is preferred that theresponses to the excitation pulses are summed.

For simplicity and economy, it is preferred that the application anddetection steps take place in the absence of an applied magnetic field(e.g. non-radio frequency magnetic field).

Preferably, the interval between the first and second pulses of theseries is different from the interval between the second and thirdpulses. It has now been discovered that, using such intervals,stimulated NQR echoes can be generated, with the attendant possibilityof there being more echoes generated and detected than there are pulsesapplied. This can in turn afford the advantage of larger signal-to-noiseratio and increased sensitivity mentioned previously.

The pulse times τ_(n) are preferably chosen to have no common factor andso that the echo times are distinct and separated from the excitationpulse times. Preferably τ_(n) >2τ_(n-1) and in some cases it may benecessary to have the condition 2τ_(N) <T₁ where T₁ is the spin-latticerelaxation time for the resonance being tested. The time intervals(τ_(n) -τ_(n-1)) may be greater than those used in conventional NQR testpulse sequences; usually (τ_(n) -τ_(n-1))>T₂ *, where T₂ * is the f.i.d.time.

In a preferred embodiment, the pulses of the series are applied at times0, τ₁ and τ₂ and the echo response signals are measured in periodsembracing at least some of the times 2τ₁, τ₁ +τ₂, 2τ₂ and 2τ₂ -2τ₁. Itis also preferred that τ₂ >2τ₁ and τ₁ and τ₂ have values which make thetimes τ₁, τ₂, 2τ₁, τ₁ +τ₂, 2τ₂ -2τ₁ and 2τ₂ separate and distinct fromone another.

If pulses are applied to the sample at times 0, τ₁ and τ₂, it has beenfound that there may be little or no substantial echo response at time2τ₁ -τ₁ for integral spin quantum number systems. This fact can be usedeffectively to perpetuate or at least prolong the echo train. A furtherexcitation pulse may be applied at time 2τ₂ -τ₁. This will producefurther echoes without suppressing or affecting the quality ofdetectable NQR information. Additional pulses may be applied at some orall of times (2τ_(n) -τ_(n-1)) (n=3, 4, 5 . . . ) to further prolong themultiple echo train. There may be times other than (2τ_(n) -τ_(n-1))when there are no substantial echo responses. In this case, the furtherexcitation pulses could be applied at these times additionally orinstead.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the invention will now be described with referenceto the accompanying drawings, in which:

FIG. 1 is a block circuit diagram of apparatus for NQR testing; and

FIG. 2 is a graphical diagram showing response signals detected in atest with the apparatus of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 a radiofrequency source 10 is connected to an rf poweramplifier 11. The output of rf amplifier 11 is connected throughswitching and timing circuits 12 to r.f. coils 13 which are disposedabout the sample to be tested (not shown). The switching and timingcircuits 12 also connect the coils 13 to a detecting and measuringcircuit 14, which is also connected to a graphical recorder 15. Theswitching and timing circuits 12 also control the timing and phase ofthe signals applied by the r.f. source 10 to the r.f. power amplifier11. It will be appreciated that numerous modifications to the apparatusdescribed are possible, as will be apparent to the skilled person.

In FIG. 2 waveform (a) represents the excitation pulse control signalsapplied to control a series of three single frequency excitation pulses,and waveform (b) represents the graphical recorder trace of detected NQRresponses. A time scale is shown below this trace. As shown, theexcitation pulses occurred at times τ=0, 2.5 ms (τ₁) and 9.5 ms (τ₂).The pulse sequence was of the form 90°_(o) -τ₁ -90°_(o) -τ₂ -90°_(o) -.Each excitation pulse lasted for 20 μs and was followed by a blankingperiod of 200 μs; during these times the detecting circuit was notconnected to the coils, to avoid overload. In this particular test thesingle radiofrequency f_(o) of the excitation pulses (5307 KHz) wasslightly offset from one of the resonance frequencies f_(r) =5302 kHz of¹⁴ N nuclei in the sample, which was a sample quantity of the explosiveHMX at a temperature close to 298K. This produces a response showingvariations at the beat frequency f_(o) -f_(r). The trace clearly showsthe NQR echo responses occurring around the times 2τ₁, τ₂ +τ₁, 2τ₂ -2τ₁,and 2τ₂. A possible response at 2τ₂ -τ₁ is either very weak or isthought perhaps not to exist at all. Parts 20 and 21 of the trace showthe f.i.d.s after the first and third excitation pulses; the f.i.d.after the second pulse does not appear on this trace which shows theoutput of only one of two channels arranged to measure signals ofopposite phase. The excitation signals in the second pulse may be inantiphase relationship with the signals of the first and third pulses.

From this trace it is clear that the three pulses generate at least foursubstantial echo reponse signals which in this instance are comparablewith the largest measurable f.i.d. It will be noted that each of theecho responses is spread over a period of about 1 millisecond which isabout twice the f.i.d. time constant T₂ * and is fifty times longer thanthe excitation pulse length. The strength and duration of the echoresponses are so great that by using them in addition to the f.i.d.'sthe sensitivity of the tests can be greatly increased.

It will be understood that the present invention has been describedabove purely by way of example, and modifications of detail can be madewithin the scope of the invention.

We claim:
 1. A method of NQR testing which includes the stepsof:applying to a sample in which selected nuclei have an integral spinquantum number, a series of at least three excitation pulses ofelectromagnetic waves at a single radiofrequency to excite quadrupoleresonance in said selected nuclei; and detecting echo response signalsfrom said sample at a plurality of times; said series of at least threeexcitation pulses being arranged in time to generate a greater number ofsaid echo response signals in a predetermined substance than a number ofsaid at least three excitation pulses.
 2. A method as claimed in claim1, wherein said echo response signals are summed.
 3. A method as claimedin claim 1, wherein said step of applying said series of at least threeexcitation pulses and said step of detecting said echo response signalsfrom said sample take place in an absence of any appliednon-radiofrequency magnetic field.
 4. A method as claimed in claim 1,wherein a first interval between a first pulse of said series of atleast three excitation pulses and a second pulse of said series of atleast three excitation pulses is different from a second intervalbetween said second pulse of said series of at least three excitationpulses and a third pulse of said series of at least three excitationpulses.
 5. A method as claimed in claim 1, wherein:said step of applyingsaid series of at least three excitation pulses includes a step ofapplying three pulses at times 0, τ₁ and τ₂, respectively; and said echoresponse signals are detected in periods embracing at least one of times2τ₂, τ₁ +τ₂, 2τ₂ and 2τ₂ -2τ₁.
 6. A method as claimed in claim 5,wherein:τ₂ >2τ₁ ; and τ₁, τ₂, 2τ₁, τ₁ +τ₂, 2τ₂ -2τ₁ and 2τ₂ are timesseparate and distinct from one another.
 7. A method as claimed in claim5, wherein τ₁ is greater than one millisecond.
 8. A method as claimed inclaim 1, wherein:said pulses of said series of at least three excitationpulses are applied at times τ_(n), where n=0 to N and N is greater thanor equal to two; and at least one of said pulses of said series of atleast three excitation pulses being applied at a respective time 2τ_(m)-τ_(m-1), with m being greater than one for such respective time.
 9. Amethod as claimed in claim 5, wherein a further pulse in said series ofat least three excitation pulses is applied at a time 2τ_(n) -τ_(n-1), nbeing greater than one.
 10. A method as claimed in claim 1, wherein,each of said series of at least three excitation pulses which isinterposed between two other adjacent pulses of said series of at leastthree excitation pulses is not equally spaced between said two otheradjacent pulses.
 11. A method as claimed in claim 1, wherein:said stepof applying said series of at least three excitation pulses includes astep of applying three pulses at times 0, τ₁ and τ₂, respectively; andsaid echo response signals are detected in periods embracing at leastone of times 2τ₁, τ₁ +τ₂, 2τ₂ and 2τ₂ -2τ₁.
 12. A method as claimed inclaim 5, wherein a further pulse in said series of at least threeexcitation pulses is applied at time 2τ₂ -τ₁.
 13. A method of NQRtesting which includes the steps of:applying to a sample in whichselected nuclei have an integral spin quantum number, a series of atleast three excitation pulses of electromagnetic waves at a singleradiofrequency to excite quadrupole resonance in said selected nuclei;and determining a plurality of times when echo response signals areexpected to occur in said sample; detecting echo response signals fromsaid sample at said plurality of times when echo response signals areexpected to occur; said series of at least three excitation pulses beingarranged in time to generate a greater number of said echo responsesignals in a predetermined substance than a number of said at leastthree excitation pulses.